The metals heat treating industry in the United States is a $20 billion industry. Globally, heat treating represents nearly $75 billion or more in value added to manufacturing. The process of carburizing alloys is estimated to account for about twenty percent of heat treating activities. Carburized parts are essential to the fabrication and operation of components in transportation, aerospace, defense, construction, chemical and materials, industrial processing and heavy equipment industries.
Carburizing is a surface treatment which produces a hard surface on steels due to the reaction of carbon containing gases with the alloy surface and diffusion of carbon into the alloy to form a hardened surface layer commonly referred to as a case. In a typical carburizing process, austenitic steel is brought into contact with an atmosphere having sufficient carbon potential to cause absorption of carbon at the alloy surface and diffusion of carbon into the alloy to create a carbon concentration gradient between the alloy surface and interior [see Carburizing and Carbonitriding, American Society for Metals (Metals Park, Ohio) 1977; Metals Handbook: Heat Treating, vol. 4, 9th ed., American Society for Metals (Metals Park, Ohio) 1981, p 135, p. 417–431; W. Hume-Rothery, The Structures of Alloys of Iron, Pergamon Press (Oxford, England) 1966]. The shape and depth of the carbon concentration profile in the surface layer depends largely on the duration of the carburizing cycle, the temperature of the cycle, and the carbon potential in the atmosphere. The carbon concentration gradient and subsequent quenching process determine the final structure and properties of the carburized surface layer.
The development of conventional carburizing process sensors, computerized control systems, and on-line process modeling have increased the quality of heat treating with respect to accuracy and reproducibility of carburizing processes and case hardened alloys [see B. Edenhofer, “Carburizing and Nitriding Industry in the Eastern Hemisphere”. Proceedings of the Second International Conference on Carburizing and Nitriding with Atmospheres, 6–8 Dec. 1985, Cleveland, Ohio. ASM, 1985, Page 3–8; Z. Wang, and J. Zhang, “Precise Control of Gas Carburizing Process by Microprocessor”. Proceedings of International Heat Treating Conference: Equipment and Processes, 18–20 Apr. 1994, Schaumberg, Ill. p. 479–482. ASM 1994; Karlo Raie, “Control of Gas Carburizing by the Diagram Method”. Scandinavian Journal of Metallurgy (1993), 22, p. 50–54; T. Reti, M. Reger, and M. Gergely, “Computer Prediction of Process Parameters of Two-Stage Gas Carburizing”. J. Heat Treating, Vol. 8, No.1, 1990, p. 55–61; T. Guler, “Optimizing Gas Carburizing Atmospheres with a Supervisory On-Line Carbon Diffusion Control System.” Industrial Heating. January 1997. p. 31–34].
Since their introductions in the early 1970's, oxygen sensors or probes have become the tool of choice for control of carbon potential in carburizing applications [see U.S. Pat. Nos. 3,454,486, 3,546,086, 3,596,345, 4,101,404, 4,193,857, 4,588,493; British Patent 4,101,404;] [B. Edenhofer, Proceedings of the Second International Conference on Carburizing and Nitriding with Atmospheres, 6–8 Dec. 1985, Cleveland, Ohio. ASM, 1985, p. 3–8; D. W. McCurdy, “Improving the Accuracy of Oxygen Probe Control Systems”. Proceedings of International Heat Treating Conference: Equipment and Processes, ASM. 18–20 Apr. 1994. Schaumberg, Ill., p. 117–121]. These conventional sensors provide measurements of the carbon potential in the atmosphere by measuring the oxygen partial pressure and converting it into carbon potential. This capability has introduced significant improvement in the control of carburizing processes. However, due to their reliance on gas phase measurements and subsequent conversion to carbon potentials these sensors are susceptible to problems with carbon sooting, catalytic effects, and oxygen reference potential problems, leading to undetected drift and poor failure detection [see D. W. McCurdy, Proceedings of International Heat Treating Conference: Equipment and Processes, ASM. 18–20 Apr. 1994. Schaumberg, Ill., p. 117–121; M. Howes, Proceedings of the Second International Conference on Carburizing and Nitriding with Atmospheres, 6–8 Dec. 1985, Cleveland, Ohio. Pages 9–13. ASM 1985.; R. N. Blumenthal, “A Technical Presentation of the Factors Affecting the Accuracy of Carbon/Oxygen Probes.” ASM's 1995 Conference Proceedings on Carburizing and Nitriding with Atmospheres. Cleveland, Ohio, p. 17–22]. Additionally, although these probes may measure the oxygen potential in a furnace atmosphere accurately and convert this to carbon potential, this is an indirect measurement of the actual carbon potential and no information is provided regarding the actual carbon concentration or concentration profile in the alloy. As a direct result, over carburization of alloys by 10 to 15% is a commonly encountered practice in commercial processes. Consequently, inaccurate control of the carbon profile in the surface layers of processed alloys leads to loss of parts due to treated components which fail to meet specific case hardened alloy specifications.
It is most advantageous for the heat treating industry to have the means to establish and maintain appropriate process parameters for both the introduction and transport of carbon into alloy surfaces during carburization for proper process control to produce desirable carbon concentration profiles [see T. Reti, M. Reger and M. Gergely, “Computer Prediction of Process Parameters of Two-Stage Gas Carburizing”. J. Heat Treating, Vol. 8, No.1, 1990, p. 55–61; R. Fincken, “Selecting Process Controls.” Advanced Materials & Processes, vol. 155, No. 6, June 1999, p. H39–H41]. Initially, during a carburization boost stage, carbon is adsorbed in the surface layer of the processed alloy using a high carbon potential in the furnace atmosphere. In a subsequent diffuse stage, the furnace temperature and carbon potential are lowered and carbon diffuses into the steel to create a carburized surface layer. These combined treatments produce a finite and distinct carbon concentration profile in the surface layer of the processed alloy. Unfortunately, the process control of the resultant carbon profiles is significantly compromised due to the lack of a real-time direct measurements of the actual carbon profiles in the processed alloy and the inability to accurately determine the appropriate transition point between the boost and the diffuse stages [see B. Edenhofer, “Process Control of Gas Carburizing Heat Treatment of Metals”, Heft 4, 1985, p. 87–91; J. J. Bausch, L. G. Chedid, and R. D. Sisson, “The Design of a Fully Automated Control System for Gas Carburization.” ASM's 1995 Conference Proceedings on Carburizing and Nitriding with Atmospheres. Cleveland, Ohio, p. 23–28].
Due to the shortcomings of conventional gas phase sensors in measurement and control of carbon concentration profiles in carburization processes, a number of workers have evaluated alternative devices for measuring carbon concentration profiles in alloy materials. Moorthy, et al. reported the use of Magnetic Barkhausen Noise (MBN) measurements for room temperature measurement of the thickness of case hardened surfaces [see V. Moorthy, et al. “Evaluation of Carburization Depth in Service Exposed Ferritic Steel Using Magnetic Barkhausen Noise Analysis.” Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8–13 Dec. 1996. Vol.3, p. 1639–1642]. Moorthy found that this method was unsuitable for measurement of carbon profiles in alloys due to the individual signal contributions, noise and interference from alloy microstructural features such as grain size, grain boundaries, carbide precipitates, inclusions and dislocations. While Moorthy found that variation in carbon content at various depths significantly influences MBN levels, due to the extensive carbide precipitation created by carbon diffusion into alloy samples and the dominance of the mean critical fields produced by these precipitates, Moorthy found that the MBN technique was better suited to measurement of hardness profiles in processed alloys.
Ultrasonic techniques for measuring alloy microstructural features such as case hardness depth have been evaluated by a number of workers [see B. R. Tittman, “Determination of Physical Property Gradients from Measured Surface Wave Dispersion.” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control. September 1987, p. 500–507; B. R. Tittman, et al., “Measurement of Physical Property Gradients with Elastic Surface Wave Dispersion.” Proceedings of the 9th Symposium on NDE. Apr. 25–27, 1973, p. 20–28; C. W. Richards, Engineering Materials Science. 6th printing. Wadsworth Publishing Company, Inc., 1968; N. Borzorg-Grayeli, “Acoustic Nondestructive Evaluation of Microstructure.” Ph.D. Thesis, Stanford University, 1981; M. Pancholy, et al., “Ultrasonic Attenuation in Carbon Steels.” Indian Journal of Technology. 19(12):493–498 (1981); A. Idris, et al., “Acoustic Wave: Measurements at Elevated Temperature Using Pulsed Laser Generator and an Electromagnetic Acoustic Transducer Detector.” Nondestructive Testing and Evaluation. Vol. 11. No. 4. 1994, p. 195–213; D. R. Mitra “Case Depth Evaluation of Carburized Specimens Using Ultrasonic Methods.” B. S. and M. Sc. Thesis, MIT, 1993]. Such methods are rather limited for carbon profile measurements in steels due to the significant ultrasonic scattering and attenuation observed with austenitic structures and the anisotropic properties in heat treated alloys [see M. Pancholy, et al., Indian Journal of Technology. 19(12):493–498 (1981)]. Perhaps the most successful of these acoustical methods was that of Mitra who employed both contact transducers and non-contact, electromagnetic acoustic transducers (EMAT) to measure Raleigh wave velocities at varying frequencies in case hardened alloy samples. While Mitra was able to accurately correlate measured Raleigh wave velocities with case depth at low frequencies with his EMAT method, the technique could not provide reliable measurements of carbon concentration profiles.
Due to the limitations of such conventional carbon sensors and sensing methods for accurately determining carbon concentration profiles in carburized alloys at temperature during carburizing heat treatments, there is a pressing need to develop a reliable, real-time, direct measurement device and method for accurate measurement and control of carbon concentration profiles during carburizing heat treatments of alloy materials. Such a device, when employed with a properly designed process control system could significantly reduce or eliminate problems with over carburization in current commercial processes and provide a substantial cost savings for the heat treating industry.
It would be particularly advantageous if a measurement device, system and method were available which could directly measure a physical property profile which is created by a carbon gradient to provide measurement of carbon concentration profiles in carburized alloys during processing. Thus, a carbon sensor probe, measurement system and measurement method of the present invention, which employs electrical resistivity profile measurements for determining carbon concentration profiles in processed alloys during processing, appear to offer a heretofore unrecognized, unappreciated and unrealized solution to non-destructive, direct, real-time carbon profile measurements of processed alloys.